https://bugs.llvm.org/show_bug.cgi?id=39092
Bug ID: 39092
Summary: Inefficient lowering of constant, negative, anyext
return values
Product: libraries
Version: trunk
Hardware: PC
OS: All
Status: NEW
Severity: enhancement
Priority: P
Component: Common Code Generator Code
Assignee: unassignedb...@nondot.org
Reporter: a...@lowrisc.org
CC: llvm-bugs@lists.llvm.org
Consider a function returning a constant negative value, e.g.
define i16 @neg_const() nounwind {
ret i16 -2047
}
Compile this on a target for which i16 is not a legal type, e.g. PowerPC. For
llc -mtriple=powerpc, the following assembly is generated:
lis 3, 0
ori 3, 3, 65436
blr
We'd rather see:
li 3, -100
blr
The issue is that SelectionDAGBuilder::visitRet will call
SelectionDAG::getCopyToParts with ExtendKind = ISD::ANY_EXTEND. This calls
SelectionDAG::getNode with ISD::ANY_EXTEND, which will recognise it has a
constant argument and choose to zero-extend it. This generates the i32 constant
65436 (i.e. zext of int16(-100)) rather than the preferable -100.
By the time target instruction selection code is reached, there is no way to
know that a sign-extended form of the original constant could have been used.
As a quick experiment I changed the constant-folding code in
SelectionDAG::getNode so it performs a sign-extend for ANY_EXTEND input. That
fixes this particular issue as expected but leads to incorrect codegen
elsewhere, though I haven't yet had a chance to explore in more detail.
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